Gas sensing probes have been known which include the selectively pervious gas membrane closing the gas sensing port of an electrolyte housing that also includes a cathode and an anode extending into the electrolyte. Such sensing probes are frequently used to monitor the level of oxygen in anesthesia equipment. U.S. Pat. Nos. 3,351,544 and 3,826,730 describe the general background use of such sensing probes in which a cathode is placed very close to a membrane that is selectively pervious to the gas being tested. The gas passing the membrane into the electrolyte chamber participates in an electrochemical reaction manifested by an electric current flow between polarized electrodes. The current varies as a function of the gas's partial pressure and can therefore measure its concentration.
In the past, these sensing probes have had very short lives and had to be reconstructed quite often because the anode became corroded or the spacing between the cathode and membrane, which might be as small as 0.001 inch, became clogged or altered. Thus, there was a problem in cleaning or replacing the cathode and adding new electrolyte and membrane substructures to these sensors. U.S. Pat. No. 3,351,544 required a complicated dismantling of the electrolyte chamber to change the cathode and membrane.
U.S. Pat. No. 3,826,730 discloses a reusable connector with a long depending anode that is pushed through a top opening in a disposable electrolyte housing. Cathode connection between the reusable connector and the electrolyte housing is made through a conductive screw.
The probe described in U.S. Pat. No. 3,826,730 had very serious shortcomings in that its storage and useful life was very short. Such probes carried a warning to use before a certain date, specified as 3 months after shipment. The reason for this short life was the dehydration and/or contamination of the electrolyte. The electrolyte was in a form of a gel, but during assembly of the reusable connector and disposable electrolyte housing the seams were not always air-tight, and could permit the electrolyte to evaporate over a period of time from the orifice through which the anode extended into the electrolyte chamber of the disposable housing.
A different type probe is described in U.S. Pat. No. 3,476,672 which uses a single electrode in a half cell for sensing the ion concentrations in solutions, such as a pH meter. Such a cell with a single electrode in the electrolyte would be inoperative for sensing gas concentrations which require an electrolyte with both a cathode and an anode in contact with the electrolyte.